This invention relates to a process for preparing organohalosilanes by the so-called Rochow reaction and a copper catalyst used therein.
The Rochow reaction is typically employed in the industrial process for the synthesis of organohalosilanes such as methylchlorosilanes. The Rochow reaction is the direct reaction of organic halides such as alkyl halides and phenyl halides with metallic silicon particles which is carried out at 250 to 500xc2x0 C. in the presence of a copper catalyst. While this reaction requires to keep a high reaction rate, a key technology in the synthesis of methylchlorosilanes is to increase the selectivity of the most desirable dimethyldichlorosilane. A key technology in the synthesis of phenylsilanes is to produce the most desirable diphenyldichlorosilane and phenyltrichlorosilane in a composition matching with their demand.
More particularly, organohalosilanes are synthesized by the Rochow reaction which is gas-phase direct reaction involving passing an organic halide gas such as methyl chloride through a contact mass consisting of metallic silicon, a copper catalyst and a minor amount of a co-catalyst. Since the cost of metallic silicon accounts for a substantial proportion of the raw material cost, it is important for this reaction to take place at an increased reaction rate of metallic silicon. Since a variety of by-products usually form in this reaction in addition to the main product diorganodichlorosilane, it is also important to control the reaction conditions to form the by-products in such a proportion as to comply with the supply/demand balance of organochlorosilanes. Industrially, the reaction is generally carried out in a reactor such as a fluidized bed, vibrating fluidized bed or agitating fluidized bed while replenishing the contact mass to the reaction system. It is quite important to effect reaction while reducing the activation time (that is the time taken for activation until the reaction reaches a steady state), preventing a lowering of activity with the progress of reaction due to deposition of deactivated contact mass, that is, preventing a decline of reaction rate and selectivity, and minimizing the increase of reactor residues (high boiling fractions such as disilanes) which are essentially unnecessary components.
However, the conventional Rochow reaction requires a very long time for activation until the reaction reaches a steady state. The steady state, in turn, is relatively short. The contact mass""s activity lowers with the lapse of time, and the yield of diorganodichlorosilane decreases accordingly. In the synthesis of methylchlorosilanes, for example, there arise problems that high-boiling fractions such as disilanes and undesired products such as methyltrichlorosilane increase due to side reaction. This necessitates to exchange the contact mass in the reactor. In order to shorten the initial activation time among the above-described many factors, it is known effective to add a copper catalyst after only metallic silicon powder is preheated to nearly the reaction temperature, thereby preventing the sintering of the copper catalyst by thermal history (see JP-A 10-309465).
When reaction is carried out by this procedure, unfortunately, the results of reaction vary over a wide range. It is desired to solve the problem of process variances.
The inventor investigated why the results of reaction vary over a wide range even when reaction is carried out under identical conditions. It has been found that the activity of the copper catalyst used in the relevant reaction is largely dependent on the magnitude of strain energy of the crystal lattice that the copper catalyst possesses (simply referred to as strain energy, hereinafter) and that a large quantity of strain energy and a large surface area are essential requirements for the copper catalyst to exert a high activity. More particularly, in this reaction, the copper catalyst is mixed with metallic silicon powder and acts on an alkyl halide (e.g., methyl chloride) or aryl halide (e.g., phenyl chloride) to help produce a corresponding organochlorosilane. The reactivity largely depends on the activity of the copper catalyst. Since this reaction is basically a gas-solid heterogeneous reaction between the organic halide which becomes gaseous at high temperatures and the copper catalyst which remains solid even at high temperatures, it is fully expectable that the surface activity of the copper catalyst is crucial. Nevertheless, not only such action of the copper catalyst itself, but also the necessary characteristics of the copper catalyst are unknown. When copper oxide is used as the catalyst in this reaction, it is already known from JP-A 9-173844, U.S. Pat. Nos. 4,520,130 and 4,504,597 that the strain energy in the copper oxide powder is crucial to the activity thereof. In these patents, the concept of strain energy is merely described, the illustrative method of measuring strain energy is not described, and no reference is made to metallic copper catalysts. With the metallic copper catalyst, the process of these patents cannot be industrially carried out.
The inventor has found that the above-mentioned problems can be solved by using a metallic copper powder having a large quantity of strain.energy and hence, a highly active surface as the metallic copper catalyst for the Rochow reaction of synthesizing organohalosilanes which is capable of maintaining a high activity in a stable manner. In particular, use is made of a metallic copper catalyst in which the strain energy of crystal lattices it possesses is relaxed at a temperature below 300xc2x0 C. and which has a specific surface area of 0.05 to 2 m2/g as measured by the BET method or air-permeability method. Alternatively, use is made of a metallic copper catalyst in which when heated in air, the surface is rapidly oxidized in unison with the relaxation of the strain energy, the heat generation start temperature as measured by air-flow differential thermal analysis (DTA) is below 300xc2x0 C., and the calorific value produced is 1 to 80 cal/g. The use of the above-defined metallic copper catalyst solves the problems of prior art Rochow reaction, increases the selectivity of desired diorganodihalosilane without a variation in reaction results, and eventually improves the reaction results. The present invention is predicated on this finding.
Accordingly, the invention in one aspect provides a metallic copper catalyst for use in the synthesis of organohalosilanes, comprising a thermally active metallic copper powder having a large quantity of strain energy. In another aspect, the invention provides a method for preparing an organohalosilane by reacting an organic halide with metallic silicon particles in the presence of the metallic copper catalyst defined above. In one preferred embodiment, a copper foil powder, stamped copper powder or microscopic copper powder is used as the metallic copper powder. The copper catalyst should preferably have a specific surface area of 0.05 to 2 m2/g as measured by the BET method or air-permeability method. The copper catalyst should preferably have a heat generation start temperature of up to 300xc2x0 C. and produce a calorific value of 1 to 80 cal/g as measured by air-flow differential thermal analysis.
The metallic copper catalyst used herein is a metallic copper powder having a large quantity of strain energy, for example, ground powder or chopped powder of rolled metallic copper foil, stamped copper powder obtained by drawing and grinding rolled copper foil or machined copper plates as by stamping, or microscopic copper fines. When copper powder of this type is heated in air, the strain energy is relaxed all of a sudden at a temperature below 300xc2x0 C., rapid oxidation of the surface occurs concomitantly, and substantial heat generation is observed. In connection with the Rochow reaction, that is, the organohalosilane synthesis reaction between an alkyl halide (e.g., methyl chloride) or aryl halide (e.g., benzene chloride) and metallic silicon in the presence of a copper catalyst and a co-catalyst, the present invention uses the above-defined catalyst to solve the problem of the prior art technology that the activation time (or induction period) taken until the reaction rate and selectivity of silane synthesis reach a steady state is very long while the steady state continues relatively short.